The present invention relates to a method for carrying out the agglomeration of ore on a sintering grate, and especially for the agglomeration of iron ore to be included in the charge of shaft furnaces such as blast furnaces.
The production of agglomerates on a sintering grate usually comprises the following steps:
Preparation of a mixture of ore, solid fuel (such as coke), fines, various additives (such as fluxes), and water; PA0 Formation of a bed or layer of this mixture on the grate, being either fixed or given a translation motion, a vacuum being applied to its lower side; PA0 Ignition of the bed of mixture at its upper surface, which results in a combustion front passing downwards through the whole bed; and PA0 Unloading of the agglomerate (sinter). PA0 The characteristics of the ores and accompanying materials: it is advantageous to choose suitable materials; PA0 The proportion of return fines in the mixture: permeability is increased when the level of return fines is increased, but the quantity of usable agglomerate decreases; accordingly there is an optimum proportion of return fines in the mixture, at which the productivity is maximized; PA0 The method of preparing the mixture, and in particular the way in which water is added: the kinds of mixers, grinders, etc. have to be carefully chosen; PA0 The way in which the mixture is charged onto the grate: it is important to reduce any phenomenon causing segregation or compaction (e.g., dropping);and PA0 The regulation of the amount of water added to the mixture; we have already proposed a method of controlling the permeability by regulating the rate of addition of water to the mixture before the mixture is charged onto the grate, in order to obtain the optimum permeability of the sinter bed, i.e., the highest possible permeability consistent with the required quality for the agglomerate. PA0 a. a carriage arranged to move along a path parallel to the sinter strand, at least upstream of the place where the sintered bed is ignited, the carriage being above the strand, upstream of the ignition point; PA0 b. a moving frame having a substantially horizontal base in which a number of holes are formed, the frame being supported by the carriage and being slidable, preferably vertically, in suitable grooves formed in the carriage; PA0 c. elongate piercing members or perforating points (in the form of pins or rods, for example) designed to be placed in the holes in the base of the moving carriage, each point having an enlarged upper end or head which prevents withdrawal of the point from the frame; PA0 d. means for causing the frame to slide vertically in the carriage; and PA0 e. means (such as weights or springs) for maintaining the enlarged upper ends or heads of the perforating points in engagement with the base of the frame as the points pierce the bed.
When the grate is given translational motion it may form part of a sinter strand consisting of a series of such grates passing under an ignition hood and over a suction box.
The factors substantially affecting the sintering rate, i.e., the time spent by the combustion front in moving from the upper surface of the bed down to the grate supporting the mixture, are vacuum level, depth of the bed, and air permeability of the mixture.
The possibility of increasing the vacuum level, which results in a higher sintering rate, is limited, for the cost of the energy used by the suction fan and the cost of providing conduits and suction boxes capable of withstanding greatly reduced pressures soon become prohibitive. Furthermore, sealing problems which are difficult to overcome are encountered at very low pressures.
The depth of the bed cannot be increased above a given level, because the resulting deterioration in the quality of the agglomerate has to be compensated for by excessive coke consumption.
The permeability of the mixture on the other hand must be as high as possible but compatible with the required quality of the agglomerate.
The factors influencing this permeability are substantially as follows: